The invention relates to a method for fabricating a microcontact spring on a substrate, which can be used in particular for test connections to a semiconductor wafer.
Devices for realizing parallel contact-connection or wiring of silicon chips and test boards or printed circuit boards at the wafer level are known per se.
U.S. Pat. No. 6,184,053 discloses a method for fabricating microelectronic spring contact elements, in which at least one first layer made of metal is deposited in openings which are predetermined by masking layers on a surface of the substrate of, for example, an electronic component such as an active semiconductor component. Each spring element has a base end, a contact end and a central section. The contact end is offset in the z-direction (in height) and in the x- or y-direction relative to the base end. The method for fabricating a spring contact element on a substrate essentially comprises the steps of: depositing at least one first layer made of masking material on a surface of the substrate with openings for contact-connecting regions on the surface, depositing a layer made of a first conductive material, so that the openings are filled, restricting defined regions of the first conductive material, selectively depositing a second conductive material essentially above the defined regions of the first conductive material, and removing the first layer made of masking material, so that a self-supporting spring contact element made of the second conductive material remains.
In the case of this prior art, selectively depositing the second conductive material above the defined regions of the first conductive material proves to be difficult. Moreover, it is a dedicated method which cannot readily be incorporated in the present-day customary manufacturing sequence of semiconductor chips.
DE 198 53 445 discloses contact needles, for the fabrication of which the following steps are carried out: electrodeposition of the needle tips in the structures of a layer of resist material, electrodeposition of the spring clip attached to the needle tip in the structures of at least one further resist layer arranged on the first resist layer, and removal of the resist material. The resulting contact needle has an electrodeposited spring clip made of at least one first contact needle material and an electrodeposited needle tip made of at least one second contact material, which is arranged at an angle on the spring clip.
The method according to this prior art is optimized for oblique exposure and is thus restricted to a specific exposure geometry. Moreover, it likewise cannot readily be incorporated in the present-day customary manufacturing sequence of semiconductor chips.
A further prior art is the fabrication of so-called xe2x80x9csoft bumpsxe2x80x9d, which is investigated by various working groups worldwide. During the fabrication of the soft bumps, the resilient contact is realized by means of an elastic bump on which a metallic interconnect is situated. The disadvantage here is that the bump technology is unknown in silicon manufacturing and has to be adapted to the present-day customary manufacturing methods. Furthermore, in the event of temperature changes, the different expansion coefficients of bump and metallic interconnect cause mechanical stresses primarily in the metal interconnect, which lead to material fatigue.
It is an object of the invention to enable the cost-effective contact-connection or wiring of a plurality of silicon chips at the wafer level simultaneously.
This object is achieved by means of a method according to claim 1. The subclaims relate to preferred embodiments of the invention.
In the case of the solution according to the invention, a metal spring is realized by means of a dual damascene process. The dual damascene process is known per se in wafer manufacturing for fabricating Cu metalization tracks or Cu wirings. Therefore, it can readily be integrated into present-day customary manufacturing sequences in the fabrication of semiconductor chips.
The method according to the invention for fabricating a microcontact spring on a substrate with at least one contact pad and a first insulator layer with a window above the contact pad comprises the steps of: a) producing a via opening in a second insulator layer above a location to be contact-connected; b) producing a depression in the second insulator layer; c) filling the via opening and the depression in the second insulator layer with a metal; d) leveling the surface produced by the preceding steps, so that excess metal and insulator material are removed; e) selectively etching back a first predetermined thickness of the second insulator layer, so that the second insulator layer remains with a second predetermined thickness, so that a section of the via opening is maintained and serves as mechanical retention for the resulting microcontact spring.
In a first preferred embodiment, the location to be contact-connected is a contact pad. In an alternative preferred embodiment, a passivation layer is applied on the first insulator layer and a metallic connection is led out from the location to be contact-connected via the passivation layer in the lateral and transverse directions.
In order to improve the deposition of the metal and in order to prevent diffusion of metal into underlying layers, an electrodeposition start layer is preferably produced according to step b). In this case, the electrodeposition start layer may be constructed from at least two metal layers, a first layer being composed, in particular, of copper or nickel and a second layer serving as barrier and adhesion layer.
In particular, the leveling of excess metal and insulator material including the electrodeposition start layer is preferably effected by chemical mechanical polishing (CMP). The microcontact spring is uncovered in step e) preferably by means of an isotropic wet or dry etching step.
The second insulator layer may be a polyimide layer.
The production of a via opening in step a) is preferably carried out by means of photolithography with subsequent dry etching.
W or Ni is preferably used as metal for filling the via opening and the depression in the second insulator layer in step c).
One advantage of the invention is that no thermal stress arises in the microcontact spring, since it is a pure metal spring. A further advantage is that all the properties of the spring can be set through the choice of the method parameters: thus, the spring constant can be determined, the material can be chosen, the contact point of the spring can be predetermined, and the reproducibility during its fabrication is ensured. In particular, it is possible to fabricate springs with small base areas, and it is not necessary to lead a metalization to the passivation.
As already mentioned above, an essential feature of the invention is that known and cost-effective processes (oxide deposition, lithography, etching, CMP, PVD, micro-electrodeposition) for silicon manufacturing, which can additionally be transferred to 300 mm wafers, are used and compatibility with methods employed nowadays is thus afforded.